Bleeding during second trimester

I woke up this morning for work and used the bathroom and to my surprise I saw blood as if on my period. Immediately called my doctor and he basically put me on bed rest for a week. But he said there's no way in finding out what caused my bleeding. "Baby looks fine at the moment, and lots of movement" those were his exact words when checking on the baby.  No cramps or any pains. Anyone ever experience this? I'm about 15wks.